1. Field of the Invention
The present invention relates to a grain-oriented electrical steel sheet having a low watt loss. More particularly, the present invention relates to a grain-oriented electrical steel sheet, in which the magnetic domains are subdivided and the subdivision effect does not disappear, even if the steel sheet is subsequently heat treated. The present invention also relates to a method for producing the grain-oriented electrical steel sheet as mentioned above.
2. Description of the Related Art
The grain-oriented electrical steel sheet is used mainly as the core material of transformers and other electrical machinery and devices, and must, therefore, have excellent excitation and watt-loss characteristics. In the grain-oriented electrical steel sheet, secondary recrystallized grains are developed which have a (110) plane parallel to the rolled surface and a &lt;001&gt; axis parallel to the rolling direction. These grains have the so-called Goss texture formed by utilizing the secondary recrystallization phenomenon. Products having improved exciting and watt-loss characteristics can be produced by enhancing the orientation degree of the (110) &lt;001&gt; orientation and lessening the deviation of the &lt;001&gt; axis from the rolling direction.
Note, the enhancement of the (110) &lt;001&gt; orientation leads to a coarsening of the crystal grains and an enlargement of the magnetic domains due to a passing of domain walls through the grain boundaries. There occurs, accordingly, a phenomenon such that the watt loss cannot be lessened proportionally to enhance the orientation.
Japanese Examined Patent Publication No. 58-5968 proposes to lessen the watt loss by eliminating the nonproportional phenomenon regarding the relationship between the orientation enhancement and the watt loss-reduction. According to this proposal, a ball or the like is pressed against the surface of a finishing-annealed, grain-oriented sheet so as to form an indentation having depth of 5 .mu. or less. By this indentation, a linear, minute strain is imparted to the steel sheet, with the result that the magnetic domains are subdivided.
Japanese Examined Patent Publication No. 58-26410 proposes to form at least one mark on each of the secondary recrystallized crystal grains by means of laser-irradiation, thereby subdividing the magnetic domains and lessening the watt loss.
The materials having ultra-low watt loss can be obtained, according to the methods disclosed in the above Japanese Examined Patent Publication Nos. 59-5868 and 58-26410, by means of imparting a local minute strain to the sheet surface of a grain-oriented electrical steel sheet. Nevertheless, the watt loss-reduction effect attained in the above ultra-low watt loss materials disappears upon annealing, for example, during stress-relief annealing. For example, in the production of wound cores, the watt loss-reducing effect disappears disadvantageously after the stress-relief annealing.
It is also known that the watt loss can be lessened by refining the crystal grains. For example, Japanese Examined Patent Publication No. 59-20745 intends to lessen the watt loss by determining an average crystal-grain diameter in the range of from 1 to 6 mm.
It is also known to impart tensional force to a steel sheet to lessen the watt loss. The tensional force in the steel sheet can be generated by differing the coefficient of thermal expansion between the insulating coating and the steel sheets.
The above described refining of crystal grains and strain imparting would not attain a great reduction in watt loss.